Our own studies have now shown that isolated hearts from male and female rats differ markedly in their response to chronic exercise programs of running and swimming. Cardiac hypertrophy and improved contractility are observed in hearts from female rats trained by swimming but neither hypertrophy nor enhanced contractility are observed in hearts of female rats trained by running. In contrast, hearts from male rats trained by running or swimming do not hypertrophy but exhibit enhanced contractility. To determine whether this differential response to a physiolgical stress may be due to differences in mechanical performance of male and female hearts, the response to varying levels of preload and afterload was examined in hearts from similar age male and female rats. Our studies so far would suggest that hearts from male rats can respond better to graded levels of increasing afterload than hearts from female rats. Any firm conclusion, however, must await results from studies where heart size is similar in male and female hearts. Plans for the coming year will focus on the influence of the sex hormones on the ultimate mechanical performance of the adult myocardium.